Facets

PH345: Winter 2025

Phil Boonstra

Horse in Motion

Is there a moment where a galloping horse has all hooves off the ground?

Eadweard Muybridge, 1878. Public Domain

Chromosomes

Schematic representation of late-prophase chromosomes (1000-band stage) of man, chimpanzee, gorilla, and orangutan, arranged from left to right, respectively, to better visualize homology between the chromosomes of the great apes and the human complement.

Figure 2, Yunis and Prakash (1982)

What is this called?

[S]lice the data into parts according to one or more data dimensions, visualize each data slice separately, and then arrange the individual visualizations into a grid (Ch21, Wilke, 2019).

Called ‘small multiples’ (Tufte, 1991), ‘trellis plots’ (Becker et al., 1996), or ‘facet plots’ (Wickham, 2016).

Why do this?

Introduces a third dimension (first two dimensions being x and y). Essentially another aesthetic.

Tufte’s principles of graphical excellence:

  • Present many numbers in small space

  • Make large data sets coherent

What are guidelines?

Use when you want to focus audience attention on how differences

Faceting variable must be categorical

Each mini-plot should (normally) have same structure: common axes, scales, etc

Hadley Wickham (1979-)

New Zealand statistician, Chief Scientist at Posit PBC

Creator of ggplot2 and the tidyverse

John Chambers Award for Statistical Computing (2006); Fellow of ASA (2015); COPSS Presidents’ Award (2019)

Two flavors of faceting in ggplot2

  1. facet_wrap(): wrap a 1D ribbon of plots into 2D grid
  2. facet_grid(): create a 2D grid of plots

See Chapter 16 of ggplot2 book (https://ggplot2-book.org/facet)

We’ve already seen facet_wrap()

# Install and load datasauRus if not already installed
if(!require(datasauRus)) {install.packages("datasauRus");library(datasauRus)}
ggplot(datasaurus_dozen) +
  geom_point(aes(x = x, y = y), size = 1) + 
  facet_wrap(vars(dataset), ncol = 5) +
  guides(color = "none") +
  theme(text = element_text(size = 20))

facet_grid() doesn’t work here

# Install and load datasauRus if not already installed
if(!require(datasauRus)) {install.packages("datasauRus");library(datasauRus)}
ggplot(datasaurus_dozen) +
  geom_point(aes(x = x, y = y), size = 1) + 
  facet_grid(vars(dataset)) +
  guides(color = "none") +
  theme(text = element_text(size = 20))

Unemployment in USA

Jorge Camoes, https://excelcharts.com/charts-monthly-unemployment-rates-by-state-1976-2009/

Wrapped facet by US States, ordered by size of workforce. Easy to compare trends across states. Unemployment band provides anchors

Life expectancy in USA

Anna Maria Barry-Jester, https://fivethirtyeight.com/features/as-u-s-life-expectancies-climb-people-in-a-few-places-are-dying-younger/

Sort of a facet grid by categorized latitude and longtitude. The trend line provides an anchor. The amount of purple and orange allow to immediately determine

References

Becker, R.A., Cleveland, W.S. and Shyu, M.J., 1996. The visual design and control of trellis display. Journal of computational and Graphical Statistics, 5(2), pp.123-155.

Tufte, E.R., 1991. Envisioning information. Optometry and Vision Science, 68(4), pp.322-324.

Wickham H (2016). ggplot2: Elegant Graphics for Data Analysis. Springer-Verlag New York. ISBN 978-3-319-24277-4, https://ggplot2.tidyverse.org.

Wilke, C.O., 2019. Fundamentals of data visualization: a primer on making informative and compelling figures. O’Reilly Media.

Yunis, J.J. and Prakash, O., 1982. The origin of man: a chromosomal pictorial legacy. Science, 215(4539), pp.1525-1530.